As the use of portable devices has increased, users can now use various services such as web surfing and electronic financial transactions as well as communication between individuals and music listening using portable devices. Services, which have been provided through computers, are now also provided through portable devices to allow users to use such services regardless of time and place.
However, since the portable device provides a limited-size display in order to increase portability, there is inconvenience in using various services. Thus, a double-sided-display portable device includes a display unit at the front side and a display unit at the rear side to increase user convenience compared to the conventional portable device. Since the double-sided-display portable device includes two display units, there is a need to provide a method for unlocking a plurality of display units.